Minimally Invasive Total Hip Surgery denotes a surgical procedure that has been engineered to minimize the extent of the incision and separation of normal tissues. Its advantages include: less pain, less blood loss and faster recovery time with earlier release from the hospital. Its disadvantages include poor visibility, lost of normal landmarks and technical difficulties due to lack of exposure. These disadvantages can be countered with improved instruments and retractors. This invention describes one such system.
To appreciate the mechanical problems that the surgeon faces with Total Hip surgery, one needs to understand the advantages and disadvantages of the various surgical approaches to the hip joint.
Choice of incision is usually based on the surgeon=s training and experience. The Posterior/Posterior-Lateral incisions are superior for preparation of the femur, but decidedly hamper the correct attitude of the acetabulum reaming, which has to be a straight 40 degrees abduction and 20 degrees of adduction which favors the Anterior incision. In the Posterior incision, the femur is dislocated anterior to the acetabulum and interferes with the correct attitude of the reamer. Without releasing the Gluteus Maximus the femur cannot dislocate far enough to allow the reamer to be in the correct plane. This involves considerable soft tissue release to do the job properly.
The Anterior incision is great for acetabular reaming but difficult for preparation of the proximal femur, which has to be hyper-extended. To do so, the knee is dropped off the edge of the operative table and externally rotated to expose the femoral head and neck. In heavy people, this can be a major challenge for the surgeon. Also, the femur must be dislocated anteriorly during the reaming process because all the posterior structures are intact.
The optimal choice would be to combine both incisions to take advantage of the exposure that both offer. The Posterior incision is made first, therefore the femur can be dislocated Posteriorly for acetabular reaming using the Anterior incision. Normally, with a single anterior incision, the femur is dislocated anteriorly because of the intact posterior capsule. By using both incisions, the capsular releases are complimentary and allow for much less soft tissue release and exposure, hence the present invention uses two 3-inch incisions with excellent visibility of the task at hand.
If small incisions are to work, then the operation will have to be highly engineered with supportive instruments to the relatively blind, trusting surgeon. These instruments must facilitate the various tasks without full exposure.
The most difficult mechanical problems faced by orthopedists during the Total Hip operation are: 1) Accurate resection of the femoral neck; 2) Axial placement of the femoral stem into the intermedullary canal; 3) Accurate sizing of the femoral prosthesis; 4) Adequate visibility of the acetabulum; and 5) Proper attitude of the acetabular reamer.
1) Accurate resection of the femoral neck.
The minimal incision makes it difficult to evaluate leg length and level of femoral neck resection because only the femoral head and neck are extended from the incision. This negates all the known template guides for femoral neck resection and femoral head height. Without the template guides, a preliminary femoral neck resection would have to be made guessing at the correct plane for the femoral ante-version. This “approximate cut” can cause subsequent problems.
2) Axial placement of the femoral stem into the intermedullary canal.
A perpetual problem for orthopedists is inserting the femoral stem parallel to the intermedullary canal. This problem is aggravated by the fact that most prosthetic systems have an “abductor lever arm” less than the actual lever arm present so physicians tend to match the present lever arm (femoral neck angle) by tilting the prosthesis into “varus”, a down-ward tilt. This attitude of the femoral prosthesis can lead to lateral thigh pain and early loosening of the prosthesis.
3) Accurate sizing of the femoral prosthesis.
Traditionally, sizing of the femoral prosthesis is done by progressive broaching of the femoral canal and is usually based on the maximum size of the femoral canal down near the tip of the stem. This system of instruments is based primarily on an optimum press-fit of the proximal femur that provides optimal stability and insures proximal loading of the prosthesis minimizing stress shielding and subsequent atrophy of bone.
4) Adequate visibility of the acetabulum and 5) proper attitude of the acetabular reamer.
If small incisions are to be employed, visibility and attitude issues must be addressed.
With these mechanical problems in mind the present invention was developed. There are four unique instruments in this invention that act in concert, which are the basis of this patent application.